Coded retroreflective marks offer a number of advantages for marking articles to be handled by automated equipment. For example, in comparison to coded marks formed from a series of black and white stripes such as used to mark grocery items, retroreflective code marks may be more rapidly and reliably read; they may be read at greater distances; and they permit wider latitude in unfavorable conditions--misalignment of an article or dirtying or obscuring of a mark--without preventing sensing of the mark. Further, retroreflective code marks may conveniently be reused.
However, maximum exploitation of retroreflective code marks has been prevented by lack of convenient, accurate, and inexpensive techniques for forming coded marks. The deficiencies of existing techniques can be illustrated by one previously used system, which uses code plates formed by adhering a sheet of retroreflective material to a support member and then covering the retroreflective sheet with a metal face plate in which a desired pattern of openings has been stamped. Deficiencies of this system include the high cost of preparing each code plate; the inconvenience of maintaining an inventory of different cover plates each stamped with a different pattern of openings; and the opportunity for error, since stamping of the cover plates is usually performed at a location removed from the point of application of a mark to an article.